The ultimate objective of this research is to derive a faithful and economical representation of visual processes. The basic approach is to separate the visual processes into smaller and simpler functional units for study. The method depends on a behavioral means of exploiting a nonlinear process in the retina that partitions the visual system into separate compartments in a way that allows separate investigation of each compartment, and provides a reference point in the sequence of processes. The present proposal is to continue to exploit this and related methods. Specifically, the aims are to: measure spatially antagonistic processes at a site in the visual system close to the receptors; measure sensitivity to aperiodic spatial stimuli superimposed on background fields containing periodic stimuli like sinusoidally modulated gratings, to determine whether models derived for one stimulus domain work with stimuli outside that domain; establish whether the signal and noise that determine sensitivity can be separately measured by matching contrasts and by the slopes of the functions by which the matches are measured; determine the conditions necessary to restore the correct appearance of contrast after changes in the optical quality of the eye; and measure the components of involuntary eye movements that affect the motion of the receptors and, hence, vision.